#include <stdio.h>

void insertString(char* text, char* insert_text, int a)
{
    text += 3;
    while (*insert_text)
    {
        *text++ = *insert_text++;
    }
    *text = 0;
}

int main()
{
    char text[80] = "pep";
    insertString(text, "per", 3);
    printf("%s\n", text);
    return 0;
}